I think I heard that it's possible to get 480p RGB in a PS2/Mini combo if you're using RGsB (sync-on-green) and not plain RGBs. I have no idea whether or not this is actually true or how you'd set it up, though.
After more research, I've discovered it is indeed possible. Check this out:
http://www.beharbros.com/#!toro/cfom
That device outputs 240p, 480i, and 480p via RGB SCART from the Dreamcast. It's designed specifically for the XRGB Mini to get around the fact that the Mini lacks VGA input.
Someone should really make something similar for PS2. Or at least someone should make a simple switch which allows someone to choose from RGB out and Component out without needing to swap cables.
I also emailed Fudoh and asked him which output is superior on PS2. He seems to prefer component over RGB regardless of the source material, (including PS1 games) which was surprising to me given the RGB preference of this thread. Hopefully he does not mind me quoting his email:
"The PS2 has a little more noise in the component signal than it has in the RGB signal. But then again noise is usually only a problem with 480p and that's hard to achieve with RGB (on the PS2).... Personally I just use component for my PS2. Much easier than anything else.... For me the perfect single processor with a PS2 is this:
PS2 -> component -> matrix switch -> output 1 -> XRGB-Mini (240p/480i)
and output 2 -> YUV to HDMI converter -> directly in the TV."
Who would've thought?
P.S. Fudoh is really a nice guy. Not only is he the internet's best authority on upscaling, but he answers his emails in very timely fashion for no other reason than his desire to help people.